As @Agatha_Sicily, it may be sinful if you promote those ideas e.g a lifestyle without God is seen as good.

Otherwise there’s nothing wrong with writing about this. If it is only acceptable to write about Christian people and places then the Bible isn’t acceptable as a book because it is filled with corrupt and wicked individuals

Tread carefully. If your writing is to judge cultural differences that as you say “conform to Catholic ideas”, you may indeed be morally wrong…also, be careful not to judge those professed to be Catholic, but who you personally do not feel meet your idea of what is Catholic.

More information on wether or not it’s right or wrong since it’d depend on how you’d write on it.

It wouldn’t be inherently wrong to write about it if it’s completely neutral and you are only intending to inform and not promote (I believe, I may be wrong but I can’t think of a situation where you’d be culpable for doing what I described)

Wrong? I don’t understand this point of view. Can’t we describe the evil? Aren’t we allowed to see what it unchristian?
I probably didn’t converted when I had not read the books of Dostoevsky. His writings are full of"evil" People, but contain a Christian moral.

Question is too broad.
The way you worded this, you could be writing anything from a novel about a family of Muslim immigrants to a porn movie script.
Writing should have quality and value. If it’s necessary to portray a character as immoral or amoral for the sake of telling the story, there are plenty of ways to do that. Flannery O’Connor had plenty of characters whose behavior didn’t conform to Catholic ideals; the quality of her writing overall and the fact that she didn’t glorify immorality make that perfectly okay.